Conventionally, there is known a turbocharger including first and second exhaust scrolls independent of each other. The turbocharger is provided to change a supply of exhaust gas to the second exhaust scroll according to the amount of exhaust gas per unit time discharged from an engine.
Specifically, a flow passage switching hole for guiding exhaust gas into the second exhaust scroll is opened or closed by a flow passage switching valve. (i) A small volume (small flow rate at which to blow exhaust gas into a turbine wheel from the first exhaust scroll) is achieved by the flow passage switching valve closing the flow passage switching hole. (ii) A large volume (large flow rate at which to blow exhaust gas into the turbine wheel from both the first and second exhaust scroll) is achieved by the flow passage switching valve opening the flow passage switching hole (see, e.g., JP-A-2006-291782).
There are the following issues about the conventional technology. The flow passage switching valve opens or closes the flow passage switching hole, which is formed inside a turbine housing, by its valving element. The valving element, which opens or closes the flow passage switching hole, is attached to “inside of the turbine housing” at “a position away from an exhaust inlet of the turbine housing (connecting port of an exhaust manifold)”. Accordingly, attachment of the flow passage switching valve deteriorates, thereby increasing the costs.
Specifically, a procedure for the attachment of the valving element in the conventional technology is described below. First, a rotatable shaft for rotating the valving element is inserted into the turbine housing. Next, with the rotatable shaft remaining inserted in the turbine housing, a part of a “crimping device” or a “welding device” is inserted through an opening provided for the turbine housing. The valving element is attached to the end of the rotatable shaft (end of a valve arm) by crimping or welding. Such an attachment operation is low in working efficiency, which is a hindrance to productivity. In addition, the opening through which the part of the “crimping device” or the “welding device” is inserted is closed by a cover after crimping or after welding. The cover is fixed to the turbine housing by bolts.
There is proposed a technique below to address the above-described issues. As illustrated in FIGS. 7 and 8, a movable part 160 of a flow passage switching valve 100 including a rotatable shaft 140, a valving element 150, and a lever 190 is attached beforehand to a cover 170 which is a separate member from a turbine housing 200. The cover 170 is fixed to the turbine housing 200 by way of bolts X. Accordingly, the flow passage switching valve 100 is attached to a turbocharger (which is not a known technique).
However, the proposed technique (technique to provide the movable part 160 for the cover 170 in advance) is for fixing the cover 170 to the turbine housing 200 using bolts X. (i) The use of the bolts X causes the increased number of components and increased attachment man-hours. (ii) There is produced a processing cost for forming internal threads Y or the like at the turbine housing 200. (iii) There is needed a space for forming the internal threads Y at the turbine housing 200, thereby hindering the turbine housing 200 from being downsized. (iv) Because the cover 170 is fixed to the turbine housing 200 by use of the bolt X, there is caused an attachment tolerance of the cover 170 with respect to the turbine housing 200. Accordingly, there are increased attachment tolerances of the rotatable shaft 140 and the valve with respect to a flow passage switching hole. In addition, a positioning pin may be added as a means for improving the attachment tolerance of the cover 170 with respect to the turbine housing 200. Nevertheless, there are issues of the increased number of components and increased processing cost.
The proposed technique (technique to provide the movable part 160 for the cover 170 in advance) is for sealing a clearance between the turbine housing 200 and the cover 170 with a gasket Z. When the gasket Z is employed, there is required for the gasket Z such performance that seals high-temperature exhaust gas for a long period of time and that absorbs heat deformation of a gasket sealing surface due to high temperature. Accordingly, a material of the gasket Z becomes expensive, so that there is an issue of increased cost for implementing the proposed technique.